1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of compact size and light weight at an F-number of 1.8 to 2.0 with still as high a zoom ratio as about 8, while having a good optical performance over the entire zooming range, suited to cameras for photography, video cameras, etc.
2. Description of the Related Art:
Among the zoom lenses of a relatively high range with a large relative aperture used in still cameras or video cameras, there has been the so-called 4-unit zoom lens. This 4-unit type of zoom lens comprises, from front to rear, a first lens unit for focusing, a second lens for varying the magnification, a third lens unit for compensating for the image shift resulting from variation of the magnification, and a fourth lens unit for use in balancing the focal length of the entire system and correcting aberrations. In the 4-unit zoom lens, a configuration employed is that for the zooming purpose, two lens units, and, for the focusing purpose, another one lens unit, totaling three lens units, are made movable. For this reason, there is a tendency to have a relatively complicated mounting mechanism for such a zoom lens.
Also, when focusing to shorter object distances, the first lens unit moves toward the object side. Accordingly, admission of a sufficient off-axial light beam to enter the zoom lens results in another tendency for the diameter of the front lens members to increase largely.
For these reasons, there have been many previous proposals for zoom lenses in which a lens unit other than the first lens unit is employed in focusing, or which utilize the so-called rear focus method, in, for example, Japanese Laid-Open Patent Applications Nos. Sho 62-24213, Sho 63-123009 and Hei 2-55308.
By the way, in the above-described publications, the zoom lenses disclosed have a zoom ratio of 6 to 8 or thereabout. However, the number of constituent lens elements is relatively large. An attempt to use a smaller number of lens elements in constructing the zoom lens of a range of about 8 leads to a tendency to increase the amount of movement of the fourth lens unit from an infinitely distant object to a closest object, which in turn increases the variation of aberrations with focusing. So, this increased range of variation of aberrations becomes difficult to correct. As the related art of the present invention, there are U.S. Pat. No. 5,009,492 and U.S. patent application Ser. No. 647,495 filed on Jan. 29, 1991, now U.S. Pat. No. 5,241,421, issued on Aug. 31, 1993.